Geological History of the British Flora . 245 
an exceptionally complete record ; its entire absence from 
the older Tertiary deposits is therefore all the more striking. 
The deposits from which existing species of plants have 
been obtained in Northern Europe may be grouped roughly 
into four series ; but as the number of cold or warm waves 
that have passed since Pliocene times is still uncertain, the 
classification adopted is provisional, and may need to be 
considerably extended. The provisional grouping used in 
this paper is as follows : — 
Neolithic'— Post-Glacial, but Pre-Roman. ‘Submerged forests/ 
and alluvial or lacustrine deposits with a Temperate 
flora. Cultivated plants and weeds of cultivation appear. 
(Part of the Scottish plant-bearing strata may be of more 
recent date.) 
Late Glacial : — Lacustrine deposits with Arctic plants, above 
the latest deposits showing ice-action. 
Interglacial -. — Deposits with Temperate plants, between 
strata indicating Arctic conditions. 
Early Glacial : — Flood-loams with Arctic plants, at the 
base of the Glacial deposits of Norfolk. 
Preglacial : — Newest Pliocene deposits (Cromer Forest-bed). 
A word of warning is perhaps needed as to the use of the 
table for the determination of species as ‘ native ’ or ‘ intro- 
duced.’ The term native though convenient is misleading, 
for it is doubtful whether a single one of our flowering plants 
is really native of Britain. The whole flora has originated 
probably in other and various parts of the world. We find now 
merely the species stranded by successive waves of migration, 
which have brought together a variety of continental forms, 
some Arctic, some Southern, a few even American. These 
migrations were mainly compelled by climatic changes, though 
other agencies have played an important part. To judge 
by the evidence already obtained, though negative evidence 
does not go for much, it seems probable that a far larger 
proportion of our plants was introduced by human agency 
than has been thought. 
